As conventional relays of the above type, a structure such as shown in FIGS. 1 and 2 is known. This thermal type overload relay shown partially in these drawings is so designed to be usable in common in automatic and manual restore modes. An insulating casing 1 of the relay includes as principal components, main bimetal 2 around which heating members (not shown) of three phases are wound that are connected into a power line, a shifter 3 engaging a free end 2a of the corresponding main bimetal 2 and supported in a guide groove 1a of the casing 1 and moveable in the direction of the arrow P in response to bending of the main bimetal 2, a switching mechanism 4 disposed in the casing 1 engaging one end of the shifter 3, a contact unit 5 whose turning-on/off is controlled by the switching mechanism 4, and a restoring unit 6, inoperative when the contact unit 5 is in the automatic restore or reset mode and controllable in the case of the manual restore or reset mode. The switching mechanism 4 is composed of a release lever 8 supported rotatably by the inner wall of the casing 1 at a point W via an adjusting link unit 7 and engaging a reversing mechanism hereinafter described, a temperature compensation bimetal 9 integral with the release lever 8 and engagable at a free end thereof with the end of the shifter 3, an adjusting link 10, whose one end forming a part of the adjusting link unit 7 is coupled by a pin at a point X to the release lever 8, supported rotatably at the above-mentioned point W, a cam 11a of an adjusting dial 11 engaging the other end of the adjusting link 10, and a spring 12 for pushing the other end of the adjusting link 10 against the cam 11a. The contact unit 5 is composed of a normally open contact section 13 and a normally closed contact section 14 insulated from each other and workable at different potentials, and the reversing mechanism 15. The normally open contact section 13 is composed of a terminal 19 and a movable board 18 to which are attached a normally open side movable contact 16 and a driving board 17 made of insulating material, and a normally open side fixed contact 21 attached to the casing 1 by a terminal 20 and capable of coming into and out of contact with the normally open side movable contact 16. The normally closed contact section 14 is composed of a normally closed side movable contact 23 attached to the casing 1 via a terminal 22 actuated by the driving board 17 of the movable board 18, and a normally closed side fixed contact 25 attached to the casing 1 via a terminal 24 so as to come into and out of contact with the normally closed side movable contact 23. The contact unit 5 includes an operation display segment 26 which is supported swingably by the casing 1 with one end engaging the driving board 17 and displays the state of the normally open contact section 13 and normally closed contact section 14. The reversing mechanism 15 is composed of the movable board 18 of the foregoing normally open contact section 13 supported swingably in a V-shaped groove 19a formed in the terminal 19, and a tension spring 27 for switching of the contacts, which is stretched between the movable board 18 and a lock groove 19b of the terminal 19 and reverses the movable board 18 when it is pushed by engaging an end portion 8a of the release lever 8 and its line of action crosses a point Z of the V-shaped groove 19a. While the contact unit 5 is illustrated in the state set to the manual return mode, it is necessary in the case of the automatic return mode to push down a control rod 28 of the return unit 6 to the position illustrated by the dashed line thereby to lower the normally open side fixed contact 21.
The operation of this structure will now be described with reference also to the schematic of FIG. 2, which illustrates the principle of the contact sections. As an overcurrent flows through the heating member coiled around the main bimetal 2, heating of the heating member causes the main bimetal 2 to bend thereby to move the shifter 3 in the direction of the arrow P. Due to movement of the shifter 3, its end portion pushes the free end of the temperature compensation bimetal 9 and causes the release lever 8 integral with the member 9 to turn in the clockwise direction about the supporting point X. Due to turning of the release lever 8, its end portion 8a pushes a hook portion of the tension spring 27 of the reversing mechanism 15. When the angle formed between the direction of force Ps of the tension spring 27 and the movable board 18 crosses over a value of zero degrees (the dead point) the movable board 18 reverses to the position shown by the dot-dash line, so that the normally closed contact section 14 turns off and the normally open contact section 13 turns on. When it is desired to change the value of the switching current, the reversing point must be changed because the degree of heating of the heating member is not in accord with the extent of curving of the main bimetal 2 with respect to a variation of the working current. In this case, the relay can be adjusted by turning the adjusting dial 11 to rotate an abutting portion of the cam 11a on the other end of the adjusting link 10 about the supporting point W thereby shifting the supporting point X of the release lever 8.
As is apparent from the above, the conventional relay of the foregoing structure includes the operation display segment 26 made of insulating material exclusively for this role; the supporting point Z between the movable board 18 and the V-shaped groove 19a must have the ability of conducting and the function of reversing with a high degree of reliability; in this connection, the terminal 19 formed with the V-shaped groove 19a requires for manufacture a troublesome machining process because of its complexity in shape and relatively costly material; and the driving board 17 which is troublesome to mount (thermal caulking) must be used to ensure insulation between the normally open contact section 13 and the normally closed contact section 14. Therefore, this relay had the drawbacks that the number of parts is large, the time of assembly is long, and the manufacturing cost is high.